Detection of phosphoinositide-3-kinase, catalytic, and alpha polypeptide (PIK3CA) mutations in matched tissue and plasma samples from patients with metastatic breast cancer.

Abstract

10502 Background: The PIK3CA gene is mutated in ∼25% of breast cancer cases and has been proposed as a potential biomarker for targeted therapies. The analysis of PIK3CA mutations in tumor tissue could therefore be an important step in the treatment decisions process. In cases where tumor tissue is not available, the detection of PIK3CA mutations in circulating plasma DNA might provide a noninvasive alternative. In order to determine the feasibility of plasma-based mutation testing, we analyzed the PIK3CA mutation status in matched tissue and plasma samples from patients with metastatic breast cancer. Methods: We used a highly sensitive and quantitative digital PCR-based approach, named BEAMing, to detect and quantify the four most common PIK3CA mutations (E542K, E545K, H1047R, H1047L) in exon 9 and 20, representing ∼87% of all reported PIK3CA mutations. We included 50 cases with metastatic breast cancer (stage IV) and assessed the mutational status of DNA extracted from FFPE tissue of the primary tumor and matched plasma. Results: PIK3CA mutations were detected in 15 out of 50 tissue sections (30%). The fraction of mutant to wild-type DNA molecules (mutational load) in the tissue samples ranged from 3.9% to 44%, with a median fraction of 15%. Identical mutations could be detected in all matched plasma samples. Mutational load in the plasma samples ranged from 0.061% to 22% in the 15 PIK3CA mutant plasma samples (median: 6.6%), representing 2 to 9,906 mutant molecules per ml plasma (median: 370 molecules). None of the patients with a PIK3CA negative tumor tissue section revealed PIK3CA mutations in the plasma DNA resulting in a 100% concordance between tissue and plasma samples. Conclusions: The detection of PIK3CA mutations in plasma DNA by BEAMing can be used as an alternative approach to tissue analysis in metastatic breast cancer patients. Author Disclosure Employment or Leadership Position Consultant or Advisory Role Stock Ownership Honoraria Research Funding Expert Testimony Other Remuneration Indivumed, Inostics Indivumed, Inostics